These plants make tuberous roots, like carrots, that need to have access to oxygen, and take up a lot of space. Many find their plants seem happier when the roots fill the pot, and they probably are since in that condition there isn't enough moist, airless soil (usually all peat if bagged potting soil is used) to choke/rot the roots and they get plenty of air near the drain hole(s.) When a soil is chunky, porous, with plenty of oxygen in it, pot size is irrelevant (unless it's too small) and risk of rotting roots is very low.
This plant looks dry though. If the roots were rotting, it would have brown tips, and probably look pale, chlorotic.
Watering once a month is concerning. I think this plant is getting too dry. If you're waiting that long because the soil still seems wet for that long, I would change it for sure. Even with no plant growing in it, no pot of soil should take a month to dry (especially assuming your heat source is making the air pretty dry at this time of year.) My instinct looking at these pics is that this plant needs a drink, and to be closer to a window.
Jacquie these are hardy in the ground here. How cold did your GH get?
